In general, the present invention relates to percutaneous transluminal devices and methods which are used to treat obstructed (sclerotic) vessel lumina in humans. In particular, the present invention is an improved stent that requires low expansion pressure for deployment and improved embedding of the struts within the vessel wall.
Cardiovascular disease is commonly accepted as being one of the most serious health risks facing our society today. Diseased and obstructed coronary arteries can restrict the flow of blood and cause tissue ischemia and necrosis. While the exact etiology of sclerotic cardiovascular disease is still in question, the treatment of narrowed coronary arteries is more defined. Surgical construction of coronary artery bypass grafts (CABG) is often the method of choice when there are several diseased segments in one or multiple arteries. Conventional open heart surgery is, of course, very invasive and traumatic for patients undergoing such treatment. In many cases, less traumatic, alternative methods are available for treating cardiovascular disease percutaneously. These alternate treatment methods generally employ various types of balloons (angioplasty) or excising devices (atherectomy) to remodel or debulk diseased vessel segments. A further alternative treatment method involves percutaneous, intraluminal installation of one or more expandable, tubular stents or prostheses in sclerotic lesions. Intraluminal endovascular prosthetic grafting is an alternative to conventional vascular surgery. Intraluminal endovascular grafting involves the percutaneous insertion into a blood vessel of a tubular prosthetic graft and its delivery via a catheter to the desired location within the vascular system. The alternative approach to percutaneous revascularization is the surgical placement of vein, artery, or other by-pass segments from the aorta onto the coronary artery, requiring open heart surgery, and significant morbidity and mortality. Advantages of the percutaneous revascularization method over conventional vascular surgery include obviating the need for surgically exposing, removing, replacing, or by-passing the defective blood vessel, including heart-lung by-pass, opening the chest, and general anesthesia.
Stents or prostheses are known in the art as implants which function to maintain patency of a body lumen in humans and especially to such implants for use in blood vessels. They are typically formed from a cylindrical metal mesh which expand when internal pressure is applied. Alternatively, they can be formed of wire wrapped into a cylindrical shape. The present invention relates to an improved stent design which by its specifically configured struts can facilitate the deployment and embedment of the stent within a vessel and is constructed from a manufacturing process which provides a controlled and superior stress yield point and ultimate tensile characteristics.
Stents or prostheses can be used in a variety of tubular structures in the body including, but not limited to, arteries and veins, ureters, common bile ducts, and the like. Stents are used to expand a vascular lumen or to maintain its patency after angioplasty or atherectomy procedures, overlie an aortic dissecting aneurysm, tack dissections to the vessel wall, eliminate the risk of occlusion caused by flaps resulting from the intimal tears associated with primary interventional procedure, or prevent elastic recoil of the vessel.
Stents may be utilized after atherectomy, which excises plaque, cutting balloon angioplasty, which scores the arterial wall prior to dilatation, or standard balloon angioplasty to maintain acute and long-term patency of the vessel.
Stents may be utilized in by-pass grafts as well, to maintain vessel patency. Stents can also be used to reinforce collapsing structures in the respiratory, biliary, urological, and other tracts.
Further details of prior art stents can be found in U.S. Pat. No. 3,868,956 (Alfidi et. al.); U.S. Pat. No. 4,739,762 (Palmaz); U.S. Pat. No. 4,512,338 (Balko et. al.); U.S. Pat. No. 4,553,545 (Maass et. al.); U.S. Pat. No. 4,733,665 (Palmaz); U.S. Pat. No. 4,762,128 (Rosenbluth); U.S. Pat. No. 4,800,882 (Gianturco); U.S. Pat. No. 4,856,516 (Hillstead); U.S. Pat. No. 4,886,062 (Wiktor); U.S. Pat. No. 5,102,417 (Palmaz); U.S. Pat. No. 5,104,404 (Wolff); U.S. Pat. No. 5,192,307 (Wall); U.S. Pat. No. 5,195,984 (Schatz); U.S. Pat. No. 5,282,823 (Schwartz et. al.); U.S. Pat. No. 5,354,308 (Simon et. al.); U.S. Pat. No. 5,395,390 (Simon et. al), U.S. Pat. No. 5,421,955 (Lau et. al.); U.S. Pat. No. 5,443,496 (Schwartz et. al.); U.S. Pat. No. 5,449,373 (Pinchasik et. al.); U.S. Pat. No. 5,102,417 (Palmaz); U.S. Pat. No. 5,514,154 (Lau et. al); and U.S. Pat. No. 5,591,226 (Trerotola et. al.).
In general, it is an object of the present invention to provide a stent or prosthesis which can be readily expanded and embedded into an obstruction or vessel wall with low dilatation pressure thereby minimizing the trauma and damaged imparted to the vessel wall during deployment of the stent.
It is also an object of the present invention to utilize a specifically designed configuration of the outer strut surface to facilitate embedment of the stent structure into the obstruction and vessel wall with low dilatation pressure.
Another object of the present invention is to employ a manufacturing process which optimizes the stress-strain curve characteristics that achieves an increased yield strength and ultimate tensile strength when compared to the other non-wire prior art stents.
The present invention is directed to an expandable stent which is relatively flexible along its longitudinal axis to facilitate delivery through tortuous body lumens, but which is stiff and stable enough radially in an expanded condition to maintain the patency of a body lumen such as an artery when implanted therein. In addition, the struts of the present invention have a specific trapezoidal, triangular or reduced radii configuration projecting radially outward that functions to reduce the forces necessary to penetrate the vessel wall with the stent thereby minimizing trauma or damage imparted to the wall during deployment.
The invention generally includes a plurality of radially expandable loop elements which are relatively independent in their ability to expand and to flex relative to one another. The individual radially expandable elements of the stent (cross-section of a strut) are dimensioned such that the aspect ratio of the height to width minimizes twisting or rotation during expansion. Interconnecting elements or a backbone extends between the adjacent loop elements to provide increased stability and a preferable position for each loop to prevent warping of the stent upon the expansion thereof. The resulting stent structure is a series of radially expandable loop elements which are spaced longitudinally close enough so that the obstruction, vessel wall and any small dissections located at the treatment site of a body lumen may be dilated or pressed back into position against the lumenal wall. The outward projecting strut surface converges towards the terminal end and is configured in a trapezoidal, triangular or rounded shape to facilitate embedment of the strut into the vessel wall utilizing low dilatation pressure. The individual loop elements may bend relative to adjacent loop elements without significant deformation, cumulatively providing a stent which is flexible along its length and about its longitudinal axis but is still very stiff in the radial direction in order to resist collapse.
The presently preferred structure for the expandable loop elements which form the stent of the present invention are generally a circumferential undulating or alternating loop pattern which comprises one of the radially expandable cylindrical elements. The transverse cross-section of the undulating component of the loop element preferably has an aspect ratio of about one to one (base to height) thereby minimizing any tendency of the strut to twist when expanded. The open reticulated structure of the stent allows for a large portion of the vascular wall to be exposed to blood which can improve the healing and repair of any damaged vessel lining.
The radial expansion of the expandable cylinder deforms the undulating or alternating loop pattern thereof similar to changes in a waveform which result from decreasing the waveform""s amplitude and the frequency. Preferably, the undulating or alternation patterns of the individual loop structures are in phase with each other in order to yield uniform expansion and inhibit any crimping along its length. The expandable cylindrical structures of the stent are plastically deformed when expanded so that the stent will remain in the expanded condition and therefore they must be sufficiently rigid when expanded to prevent the compression of the struts and therefore partial or total collapse of the stent after deployment. The manufacturing process of the present stent invention utilizes optimized stress-strain curve characteristics to achieve, unlike other non-wire stent designs, improved mechanical properties throughout the stent. The optimized stress-strain curve increases both the yield strength and the ultimate tensile strength of the expanded stent increasing its resistance to structural failure (fracture) or stent crushing. During expansion of the stent, the radially projecting trapezoidal, triangular or reduced radii configuration of the struts outer surface will penetrate the obstruction and vessel wall. Due to the reduced area of the outer surface, the struts are able to pierce an obstruction or the vessel wall with relative easy thereby resulting in minimal trauma or damage to the vessel wall. In addition, this design feature of the present invention helps secure the expanded stent so that it does not move once it is implanted and furthermore, minimizes projections into the blood stream.
The elongated elements which interconnects adjacent radially expandable elements should have a transverse cross-section similar to the transverse dimensions of the undulating or alternation loop components of the radially expandable element. The interconnecting elements preferably are not a unitary structure but rather alternates sectionally along the length at various degrees around the circumference of the stent. In an alternate embodiment, the interconnecting element is a unitary structure which resembles a backbone connecting the expandable loop elements.
In a presently preferred embodiment of the invention, the stent is conveniently and easily formed by first heat-treating the mechanically hardened tubular member to achieve optimum stress-strain characteristics e.g. yield strength, elongation and ultimate tensile strength. Then, the tubular member, comprising stainless steel, platinum, gold alloy, or a gold/platinum alloy, is electro-cleaned with an appropriate solution. Once the tubular member is cleansed of contaminates, the outer surface is uniformly coated with a photo-sensitive resist. Optionally, a coupling agent may be used to facilitate the bonding of the photosensitive resist to the tubular member. The coupling agent is not essential in that some tubular member compositions bond directly to the photo-sensitive resist solution without the need for a coupling agent.
This coated tubular member is then placed in an apparatus designed to rotate the tubular member while the coated tubular member is exposed to a designated pattern of ultraviolet (UV) light. The apparatus controls the exposure of the coated tubular member by utilizing a photographic film with a specified computer generated imprinted configuration, transferring the UV light in the specified pattern to the coated tubular member. The UV light activates the photosensitive resist causing the areas where UV light is present to expose (cross-link) the photo-sensitive resist. The photo-sensitive resist forms cross links where is it exposed to the UV light thus forming a pattern of hardened and cured polymer which mimics the particular stent design surrounded by uncured polymer. The film is adaptable to virtually an unlimited number of intricate stent designs. The process from the apparatus results in the tubular member having a discrete pattern of exposed photo-sensitive material with the remaining areas having unexposed photo-sensitive resist.
The exposed tubular member is immersed in a negative resist developer for a specified period of time. The developer removes the relatively soft, uncured photo-sensitive resist polymer and leaves behind the cured photo-sensitive resist which mimics the stent pattern. Thereafter, excess developer is removed from the tubular member by rinsing with an appropriate solvent. At this time, the entire tubular member is incubated for a specified period of time, allowing the remaining photo-sensitive resist polymer to fully cure and bond to the surface of the processed tubular member.
The processed tubular member is then exposed to a electrochemical etching process which removes uncovered metal from the tubular member, resulting in the final tubular member or stent configuration. Since the tubular member has not been subjected to any process such as additional heat treatments, welding/brazing or laser cutting, the finished stent will maintain the optimized stress-strain characteristics obtained in the initial heat-treatment process.
The stent embodying features of the invention can be readily delivered to the desired lumenal location by mounting it on an expandable member of a delivery catheter, for example, a balloon or mechanical dilatation device, and passing the catheter/stent assembly through the body lumen to the site of deployment.
Other features and advantages of the present invention will become more apparent from the following detailed description of the invention. When taken in conjunction with the accompanying exemplary drawings.